It has long been known that a complete digitization of the imaging chain in microscopy would make a whole range of fundamental advantages possible. Thus, for example, very light-sensitive cameras could help to lower the necessary illumination level on the object of observation and thus reduce the thermal load. Very fast cameras can record movements or variable processes which cannot be resolved with direct visual observation. Cameras with a broad spectral band can expand the perceptible wavelength range into infrared or near-ultraviolet in order for example to make fluorescence of dyes or luminescence of particular marker substances visible. Furthermore, the number of observers could in principle be made as large as desired.
At the same time, the user increasingly expects image data to be able to be easily archived. In the case of surgical microscopes, for instance, this is also for the purpose of meeting obligations to provide proof of quality and for teaching purposes. In respect of the image representation itself, users increasingly expect images to be presented to them that are ergonomically adapted optimally to their aesthesiophysiology, for instance by targeted adaptation of contrast or colour space, or by applying suitable noise suppression or image sharpening algorithms, etc., such as is already common practice in digital consumer products (for instance HDTVs, digital photography, etc.).
A problem not solved sufficiently until now which increasingly occurs with digital image acquisition devices is that the depth of field in the object space is in principle coupled to the lateral resolution and tends to be too low for many fields of application. In the at least approximately diffraction-limited case, the object-side lateral resolution (minimum resolvable structure width) is proportional to λ/NA and the object-side longitudinal resolution (=depth of field) is proportional to λ/NA2, wherein λ is the wavelength of the light used for observation and NA is the object-side numerical aperture of the optical system.
Thus while the resolution of smaller structures requires an enlargement of the object-side numerical aperture, on the other hand the depth-of-field range in the object area is thereby strongly disproportionately reduced, namely according to a quadratic law. In devices for direct visual observation, this is mitigated by the fact that, when focusing on particular objects as carriers of image information, the eye of an observer unconsciously (by reflex) accommodates to the observed depth level in which the maximum object contrast lies. However, this accommodation no longer takes place with the digital acquisition and representation of image information, which is why the depth of field of the images presented on a display is too low for many applications.
This fundamental disadvantage has until now prevented digital microscope systems from comprehensively conquering particular fields of use in which a sufficient depth of field is crucial. This is the case for example when small objects are manipulated under constant microscopic observation. An important example of this is microscope-assisted surgery, for example ophthalmic, dental, neuro- or spinal surgery in which a sufficient depth of field is absolutely essential.
To increase the depth of field in digital image acquisition devices, various methods are already known which can be roughly distinguished according to static and dynamic methods.
A device for increasing the depth of field is known from DE 1 803 964 in which a rotating optical element is divided into sectors which are to have a different optical refractive power effect. These are introduced into the beam path periodically by rotating the element.
However, a disadvantage of this operating principle is that a rotary device for refocusing only functions for discrete z-positions, and thus corresponding discrete angular positions of the rotatable element. A continuous movement of the element would always lead to a blurriness in the imaging because a rotation-symmetrical pupil manipulation is only possible when the optical axis of the microscope and the optical axis of the optical element are located in one line of sight on the rotatable device. In all other angular positions, however, the rotatable element will induce non-rotation-symmetrical imaging errors which are not acceptable for high-quality imaging. It is therefore necessary to rotate the rotatable disc in each case by discrete angular positions, thus to accelerate it in the interim, decelerate it again and then record a partial image. In practice, this substantially limits the usable video frame rate.
In addition, the diameter of the disc is necessarily several times greater than the diameter of the microscope lens system, resulting in a bulky component which would also produce a relatively high level of disruptive mechanical vibrations as a result of the necessary acceleration and deceleration. Furthermore, only relatively few discrete z-positions of the focal position, which are also not freely adjustable, can be realized with such an element.
In DE 10 2006 025 129 A1, an arrangement is described in which a micromirror array with individually controllable micromirrors with adjustable spatial orientation is to be used to alter the focal plane. However, there are no specific embodiment examples in the published document.
A variable mirror with optical refractive power which deflects the beam path by 45° would produce strongly non-rotation-symmetrical image errors which vary with the switching state of the mirror and therefore also cannot be corrected by static optical elements. In addition, a segmentation of the pupil, which in principle cannot be avoided due to the mirror microfacets, would produce light interference which is superimposed on the useful light beam path and cannot be separated from it. Furthermore, the required micromirror matrix is an expensive and complex specialized component which is not yet available as a commercial product. Finally, with this operating principle, at least one fold of the beam path is necessary, which in general is not desired in surgical microscopes for design reasons. Here, the visual axis is to coincide as much as possible with the mechanical axis of the device, in order that a precise positioning of the video head that is as intuitive as possible for the operator is more easily possible.
Furthermore, lenses or lens groups can be shifted mechanically to adjust the focal position in the longitudinal direction, as described e.g. in US 2006/0181767 A1. A disadvantage of this is that to achieve a predetermined alteration range of the focal plane relatively large displacement paths are necessary, which cannot, or can only with difficulty, be realized mechanically in the desired frequency. Moreover, relatively large shocks and vibrations are induced which are to be seen as problematic with respect to both the positional tolerance requirements of the optical elements and the purely acoustic interference effect.
The static methods include the use of special phase- and amplitude-modulating masks in the area of the pupil of an optical system (so-called EDOF masks). By means of such methods (known under the name “PSF engineering”) the linear expansion of the central diffraction maximum of the point spread function of the optical system can be enlarged and thus the optical depth of field can be increased. On the other hand, with known PSF an image reconstruction in other z-planes can take place, by unfolding operations within certain limits, as described e.g. in U.S. Pat. No. 7,444,014 B2.
However, a disadvantage here is that such phase masks always also reduce the image contrast (which can be quantified more precisely for example by the modulation transfer function) compared with a diffraction-limited system with an identical aperture but without phase mask. In particular, the best achievable resolution within a particular plane can be made significantly worse, which is not acceptable for many applications.
Furthermore, amplitude masks absorb a certain portion of the light used for the imaging. This is also problematic in many applications, wherein a compensation for the transmission losses by a higher illumination level is often not acceptable because of the associated higher radiation and thermal load, such as e.g. in the case of ophthalmological applications. Furthermore, it is disadvantageous that, as the depth of field increases, the noise of the image sensor leads to ever greater impairment of the quality of the reconstructed image.